1. Field of Invention
Embodiments of the present invention are directed towards efficient coupling of light and, in particular, to adiabatic planar waveguide couplers and their applications.
2. Discussion of Related Art
Efficient coupling of light emitted or received by high index and high numerical aperture (NA) devices, such as organic light emitting diodes (OLEDs), light emitting diodes (LEDs) and Laser diodes, has not been available other than through discrete devices such as lenses and gratings. Such coupling is also hindered due to the lack of transparent, high index optical materials having an index n above the range of approximately 1.44 to 1.7 for such discrete devices. Transparent oxides and dielectrics are often prepared by melting or sintering of low melting precursors, for example with flame hydrolysis precursors. Highly doped glass is also used to form transparent optical films. Glass films composed of suitably transparent materials are limited to doped glass such as borophosphosilicate glass (BPSG), which can be deposited as a film and then heated to optically clarity. In general, high melting oxides have higher index of refraction and require refractory temperatures. However, such materials recrystallize upon cooling and are therefore not applicable to low loss optical applications due to increased scattering. In addition, reflection of light at an interface between a device with a first index of refraction and a second device with a different index of refraction is a further significant limitation to efficient coupling, transport, transmission and conversion of light. Coupling between adjacent optical elements with widely differing characteristics of étendue or optical size and solid angle, required discrete optical elements such as lenses, gratings or so called “photonic crystals” in order to transform the divergence or the optical size, or both, for coupling to a second device with different optical characteristics. To date, there has not been an optical coupler which was able to couple and transform in one continuous device. Consequently, it has not been possible to integrate high index film layers to form a waveguide structure having a larger index contrast.
Scientific modeling confirms that the measured efficiency of discrete lens-based coupling devices such as a single mode laser diode to an optical fiber is less than about 20-30% after optimization. Consequently, fiber coupled sources are less than about 15% efficient. Similarly, out coupling of an LED to air is less than about 30-40% efficient in production due to the high numeric aperture (NA) of diode devices, which are typically lateral wave guide devices where light is out coupled generally through the p-side window and through a transparent conductor layer. Similarly, collection from a concentrating mirror is limited to less than about 40%, even for high f-number (long distance focal point) mirrors, and is typically much less with low f-number mirrors that are more compact and cost effective concentrators. Out coupling of OLEDs, which have recently been shown to be 100% efficient internally, can result in less than 20 to 25% of the light being actually extracted.
Therefore, there is a need for better production of materials directed to coupling light into and out of discrete devices, waveguides and fiber more efficiently.